Wait a minute, you're missing a step in logic. If the feds stop regulating speed limits, who's to say that the states won't start? In fact, haven't they already started?
One state legalizies pot. Then we need drug guards on state borders.
Not at all. One state legalizing pot should help the states that don't, because all the druggies will move to the state where it's legal.
The point is that there is a federal interest in what states do, 24/7. Think about gay marriage. There is a federal interest in that too. It simply won't be sustainable to have legalized gay marriage in some states, and not others. You just watch.